well i solved it before but i lost my solution so here i coiped it from some site:
Prove that for every natural number n, (x - y) divides (x^n - y^n).
Our first step in mathematical induction is the base case; that is, for n = 1.
1) Base Case: Let n = 1. Then
(x^n - y^2) = (x - y), and (x - y) is obviously divisible by (x - y).
Therefore, the formula holds true for n = 1.
2) Induction Hypothesis: Assume the formula holds true for n = k. That is, assume that
(x - y) divides (x^k - y^k).
( We want to prove that (x - y) divides x^(k + 1) - y^(k + 1) )
But what does x^(k + 1) - y^(k + 1) equal?
x^(k + 1) - y^(k + 1)
I'm going to re-express these two terms.
(x^1)(x^k) - (y^1)(y^k)
x*(x^k) - y*(y^k)
I'm going to use a little trick, by "adding zero" in the middle.
x*(x^k) + 0 - y*(y^k)
I'm going to subtract x*(y^k) and add x*(y^k). After all, subtracting and then adding the same term is the same as adding zero.
x*(x^k) - x*(y^k) + x*(y^k) - y*(y^k)
Now I'm going to factor the first two terms and the last two terms.
x(x^k - y^k) + (y^k)(x - y)
Look closely at this;
(y^k)(x - y) is obviously divisible by (x - y).
By our induction hypothesis, we assumed that (x^k - y^k) is divisible by (x - y). Therefore, x(x^k - y^k) is divisible by (x - y).
The sum of two terms both divisible by (x - y) is also divisible by (x - y). Therefore, what we started with,
x^(k + 1) - y^(k + 1) is divisible by (x - y)
Therefore, by the principle of mathematical induction,
x^n - y^n is divisible by (x - y) for all natural numbers n.
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